I hav open sourced my 2d game engine, and I am getting ready to rebuild the project from scratch with a different direction and design. I am wondering what's the best way to get developers engaged in the project.

Also, what sort of materials do I need to provide for these potential developers? Is it similar to traditional software development where stacks of documentation is needed or is it more like a lean startup where everything is fast-paced, scrum-like and mostly word-of-mouth/review-based?

I'm just starting to get into open-source software, but I'd imagine that the best way to get people involved is to just talk to them. If there are coding groups in your area, email them or go to a meeting. Go to a hackathon and talk to people there about your idea. Keep in mind that the people you want probably have other offers, too, so you need to figure out how to convince them to work with you. If you want to work with people that aren't in your area, go online and find people (posting in this subreddit is probably a good start).

Also, yes, documentation is really nice. You should be able to hand a developer your entire project (plus documentation), and they'll never need to ask you any questions about how the current product works. That's never how it actually works out, but it's a good goal to strive for. At least a basic description of the overall organization of the project and what the different parts are and how they interact. You don't need stacks, but don't just hand them spaghetti code and expect them to figure it out (they might just leave in frustration).

I actually already use Github for my open source projects, including this one. What I am looking for is how I find contributors. Do I just look for contributors to similar projects on GH and contact them directly?

Thanks, that's what i figured, but was hoping there was another way... None of my dev friend have interest in game dev, or are already involved heavily in other projects and the local university is pretty lacking in talent (is that too harsh?).

Should i first worry about getting an mvp out there before looking for help?